1. Field of the Invention
The present invention relates to a method and apparatus for controlling charge and discharge in an energy storage system (ESS), and more particularly, to a method and apparatus which reduces the frequency of switching between charging and discharging of battery packs included in the ESS to improve the service life of the battery packs.
2. Discussion of Related Art
In recent years, according to the policy for expanding renewable energy sources in order to ensure energy at the national level, energy production facilities using wind power, solar energy, and the like have been rapidly increasing. Such new renewable energy is the key solution to the problem of depletion of fossil energy and environmental problems, and therefore countries have actively carried out research into renewable energy.
It is desirable that energy produced in power plants based on renewable energy can be stored. Since renewable energy is based on natural forces or the like, an energy production pattern of the new renewable energy does not usually coincide with a consumption pattern in actual loads, and therefore renewable energy generation power is temporarily stored and then the stored renewable energy generation power is supplied to the loads as necessary or desired.
According to this need, energy storage systems for storing amounts of energy have been developed and are commercially available. The ESS is generally used as a system that stores power supplied from a system power source or power supplied from a power plant using renewable energy to efficiently use the stored power. For example, residual energy is stored in the ESS during the early hours of the morning with relatively small energy consumption, and the stored energy is utilized later during periods of peak usage, whereby it is possible to efficiently accommodate loads.
In a typical ESS, a plurality of battery packs are connected and managed by a battery management system (BMS) for monitoring and controlling state information such as a state of charge (SoC), voltage, current, temperature, and the like of each battery pack is provided in the ESS. Meanwhile, each of the battery packs generally includes a plurality of battery cells.
FIG. 1 is a diagram illustrating a control system of a conventional energy storage system (ESS). In this figure, one energy management system (EMS) 10 controls operations of at least one ESS 20. In a smart grid and microgrid which have been recently become more prevalent, a large amount of renewable energy should be stored for a relatively short amount of time. However, a large amount of power can be supplied to loads in emergency situations, and therefore a plurality of ESSs 20 are utilized. Here, the plurality of ESSs 20 may be managed by the EMS 10.
Meanwhile, in a single ESS system, the ESS 20 includes a BMS 21 that controls charging/discharging of included plurality of battery packs 22. The BMS 21 maintains the SoCs of the plurality of battery packs 22 to be balanced. That is, when power is stored in the ESS 20, the BMS 21 controls the SoCs of the plurality of battery packs 22 to be substantially identical to each other, and even when power is discharged from the ESS 20, the BMS 21 controls the SoCs of the battery packs 22 to be substantially identical to each other.
In such a BMS, when the ESS 20 is operated in a charge mode, all of the battery packs 22 are charged, but on the other hand, when the ESS 20 is operated in a discharge mode, all of the battery packs 22 are discharged.
Accordingly, charging and discharging operations of the battery packs 22 are frequently performed. That is, switching between charging and discharging should be very frequently performed, resulting in a reduction in the service life of the battery pack and damage to the battery pack. Thus, there are demands for charging/discharging control technology for improving the service life of the battery pack and preventing damage to the battery pack in the ESS.